1. Field of the Disclosure
The disclose relates generally to asymmetric supercapacitors, and more particularly, to transition metal carbide and nitride based asymmetric supercapacitors.
2. Brief Description of Related Technology
Batteries are important energy storage devices used for military and commercial applications. While these devices can have energy densities exceeding 100 Wh/kg, this energy is difficult to fully access in pulsed and high power applications due to the relatively slow kinetics associated with the redox processes of batteries.
Supercapacitors are a class of electrochemical energy-storage devices that could complement batteries for load-leveling or uninterruptible power supply applications. Referring to FIG. 1, in terms of specific energy and specific power, supercapacitors fill the gap between conventional capacitors and batteries. The times shown in FIG. 1 are the time constants of the device, obtained by dividing the energy density by the power. Currently available supercapacitors are well suited to handle pulses of up to a few seconds. To achieve broader application, however, capacitors will have to efficiently manage longer pulses, which translates to higher energy densities.
Supercapacitors have unusually high capacitances compared to traditional capacitors, due to their charge storage mechanisms. In addition to charge storage during formation of an electrical double layer, a portion of the capacitance may be from fast, reversible redox reactions taking place near the electrode surface. Supercapacitors provide higher power than batteries, while storing less energy. Most commercial supercapacitors use very high-surface-area carbon-based active materials. These materials typically yield specific capacitances of up to 200 F/g.
Some materials exploit, fast, reversible faradaic redox reactions that occur with the first few nanometers of the surface of the active material. This pseudocapacitive mechanism has been demonstrated for materials including metal oxides and hydroxides, such as RuO2 and MnO2, and conducting polymers such as polyaniline and polypyrrole. Hydrous RuO2.xH2O is a benchmark pseudocapacitive material and has been shown to yield specific capacitances ranging from 720-1300 F/g, depending on the preparation and heat treatment conditions. Despite the high specific capacitance of the Ruthenia-based materials, their high cost makes them unattractive for large-scale use, and therefore the commercial application of Ruthenia-based supercapacitors has been limited.
Despite their proven performance benefits, supercapacitors have not found widespread commercial use, largely due to the need for higher energy densities and lower cost. For example, the United States Department of Energy has targeted energy and power densities of 15 Wh/kg and 700 W/kg, respectively, for supercapacitors to be used for load-leveling and regenerative braking in hybrid and electric vehicles. State-of-the-art symmetric supercapacitors employing high area carbon electrodes and non-aqueous electrolytes can reach energy densities of 3-5 Wh/kg with power densities of 700 W/kg. These devices have been highly optimized, and only incremental gains in energy density are expected in the future.
A type of asymmetric device architecture has been demonstrated where two different types of electrode materials, for example, a supercapacitor-type electrode and a battery-type electrode are combined. The voltage windows in these devices are wider than those for conventional symmetric supercapacitors. Potential windows as wide as 2 V have been reported for a carbon-MnO2 aqueous system, however, the energy densities were limited due to the moderate capacitances of carbons in aqueous electrolytes.